This invention relates to the field of art pertaining to beam scanning using single or multiple light beams, modulated with a signal(s) representative of information to be recorded on a recording medium, such as a photoreceptor, by scanning the surface of the recording medium with the modulated beam(s). In particular, this invention relates to the effective use of overlapping modulated beams from a solid state multi-emitter laser source in a raster output scanner (ROS).
It is well known in the scanning art to utilize a multiple laser beam source for scanning a recording medium surface simultaneously with a plurality of beam spots. The individual beams are independently modulated by information signals and the recording medium is simultaneously scanned by the modulated beams. An important aspect in multibeam scanning has been the arrangement of the beams relative to the scanning direction. With standard laser sources, the spots on the scanned surface are most easily arranged to scan well-separated lines. However, this has the disadvantage that there is a spacing between the picture elements or pixels formed on the recording medium surface so that multiple scan runs must be accomplished to fill in the inbetween element spacings. This requires a significant amount of buffer memory, resulting in additional expense, and can lead under certain circumstances to an unacceptable amount of what is known in the art as banding. One manner of decreasing the inbetween element spacing is to arrange the beams to be inclined at angles relative to the scanning direction. However, the scan start and stop points along the scanned surface will be different for all the beams so that beam detectors and timers must be provided to provide for correct beam start and stop times. An example of the foregoing is disclosed in U.S. Pat. No. 4,404,571.
What would be advantageous is the provision of a solid state multi-emitter laser with output beams that overlap in a direction perpendicular to the scanning direction at the photoreceptor surface. The problem, however, with overlapping beams is that the beams may interfere resulting in undesirable variations in resultant beam intensity and, consequently, printing uniformity. This problem has been traditionally avoided by generating two cross-polarized collimated beams from a laser source, such as single gas laser. An example of such an arrangement is shown in U.S. Pat. No. 4,525,024. This patent discloses a two beam polygon ROS system comprising two laser beams each having a different polarization characteristic, via polarization optic components, which are scanned, via a rotary polygon mirror and lens focusing system, onto the recording medium surface. In the case of spot overlap at the recording medium surface, the separation of the two differently polarized beams at the focused surface is approximately equal to the Gaussian diameter of a single spot. However, because of their cross-polarization, there is no optical interference between the beams and printing uniformity is guaranteed in all possible combinations of beam modulation at the recording medium surface.
More recently, there is U.S. Pat. No. 4,637,679 disclosing a solid state laser having multiple output beams having different polarized characteristics using a polarization beam combiner.
It is quite clear that in future ROS laser printers, the single gas laser source will be replaced by a single chip, multi-emitter semiconductor laser having two or more far field output beams. See for example, U.S. Pat. No. 4,445,125. This replacement has the advantage of replacing the complex optics of the type disclosed, for example, in U.S. Pat. No. 4,525,024, and the requirement of generating two polarized beams from a single source as well as the accompanying cost of such optic components. However, the two or more beams from the solid state laser will not be cross-polarized and, although they will not naturally be at precisely the same emission wavelength, they will oscillate in and out of their dominate wavelength within their spectral emission linewidth. Therefore, they will at times optically intefere with one another when currently operated in their ON state and fail to provide a Gaussian shaped output desirable for uniform intensity printing. So, the problem remains as to how to avoid optical beam interference and resulting variations in printing intensity without the necessity of beam polarization requiring additional optical components.
The object of this invention is to provide for overlapped Gaussian-shaped laser beams focused onto the recording medium surface that will not optically interfere without the need of further modification to the optical properties of the output beams propagating between the laser source and the recording medium surface thereby preventing any possibility of nonuniformity in printing due to optical interference.